1. Field of the Invention
The present invention relates in general to auto mode selectors, and more particularly to an auto mode selector for automatically recognizing an input/output interface such as a low voltage transistor transistor logic (referred to hereinafter as "LVTTL") or a high-speed input/output interface such as a center tapped ternminated logic (referred to hereinafter as "CTT") in a synchronous memory device without using any other option.
2. Description of the Prior Art
Generally, in a device employing input/output interfaces of two types, the interfaces must be provided in the chip by means of bonding or a metal option. However, such bonding or metal option causes the user not to change the chip of one type to that of the other type, resulting in an inconvenience in use. For this reason, the chip must automatically select one of the two modes according to external environments so that it can be made according to a user's requirement. For example, a memory employing LVTTL and CTT has a difference between the LVTTL and CTT in view of an external package pin. Namely, in the case of CTT, a reference voltage is supplied from the external pin because an input buffer is provided with a differential comparator. However, in the case of LVTTL, no reference voltage is supplied from the external pin. As a result, an internal reference voltage is used as the reference voltage of the comparator.
FIG. 1A is a circuit diagram illustrating the construction of a conventional reference voltage selector. As shown in this drawing, whether the present mode is LVTTL or CTT is discriminated using a resistor R1 connected between an external reference voltage pin Vrefp and a supply voltage source Vcc. Namely, current is supplied to the reference voltage pin Vrefp to discriminate whether the reference voltage pin Vrefp is in a floating state, and whether the present mode is LVTTL or CTT is then discriminated in accordance with the discriminated result of the reference voltage pin Vrefp. In the case of LVTTL, the reference voltage pin Vrefp is in the floating state because it is supplied with no external reference voltage. However, in the case of CTT, the reference voltage pin Vrefp has a reference voltage level because it is supplied with an external reference voltage. As a result, if Vrefp=Vcc under the condition that constant current flows to the reference voltage pin Vrefp, the present mode is decided on LVTTL due to the floating state. However, if Vrefp=Vref under the condition that constant current flows to the reference voltage pin Vrefp, the present mode is decided on CTT.
In this manner, the discrimination between LVTTL and CTT is made by comparing a voltage on the reference voltage pin Vrefp with a specified voltage. For example, a comparator may be used to compare the voltage on the reference voltage pin Vrefp with 3Vcc/4. In this case, if Vrefp=Vcc, the present mode is LVTTL. However, if Vrefp=Vref=Vcc/2, the present mode is CTT. In more detail with reference to FIG. 1A, the reference numeral 1 designates a switching circuit. In the switching circuit 1, "Vrefp" indicates an external reference voltage Vref from an external reference voltage pad, "Vref.sub.-- int" indicates an internal reference voltage Vref to be used in the case of LVTTL, and "T1" and "T2" designate switching elements for selecting one of the external and internal reference voltages Vrefp and Vref.sub.-- int according to the interface type. The reference voltage selected by the switching elements is transferred to input comparison means of an input buffer.
FIG. 1B is a circuit diagram illustrating the construction of a conventional auto mode selector. As shown in this drawing, the conventional auto mode selector comprises a differential amplifier 3 as comparison means of the auto mode selector, a comparison voltage generator 2 for generating a comparison voltage to one input terminal of the differential amplifier 3, and a flip-flop circuit 4 for generating a reference voltage selection signal Lv.sub.-- cttb in response to an output signal Outb from the differential amplifier 3 and a power-up signal Pwrup.
The operation of the conventional auto mode selector with the above-mentioned construction will hereinafter be described.
The comparison voltage Vr of 3Vcc/4 is generated by the comparison voltage generator 2 and then applied to one input terminal of the differential amplifier 3, the other input terminal of which is connected to the reference voltage pin Vrefp. In the case of CTT, Lv.sub.-- cttb=low because Vrefp=Vref and Vr=3Vcc/4. However, in the case of LVTTL, L.sub.-- cttb=high because Vrefp=Vcc and Vr=3Vcc/4. Then, the reference voltage selection signal Lv.sub.-- cttb generated by the flip-flop circuit 4 is applied to the switching elements T1 and T2 in FIG. 1A. As a result, the reference voltage selection operation is performed in such a manner that Vref=Vrefp in the case of CTT, and Vref=Vref.sub.-- int in the case of LVTTL.
However, the above-mentioned conventional auto mode selector has a disadvantage in that the consumptive flow of standby current from the supply voltage source to the ground voltage source is caused when the comparison voltage is generated. Further, in the case of CTT, when the external reference voltage is supplied to the reference voltage pin Vrefp, the amount of input leakage current becomes large due to the connection of the resistor R1 to the reference voltage pin Vrefp.